Luminaries are known that comprise a series of generally vertical, right-angle prisms for reflecting light from a centrally located lamp. The reflectors for these luminaries are made with transparent material (glass, acrylic, etc.) and typically have sets of longitudinal prisms running from top to bottom. The reflector typically has a desired overall contour provided by the series of prisms. In most cases the desired overall contour is dome-like, with an upper part of smaller diameter and a lower part of larger diameter. This configuration results in the requirement that the prisms become gradually broader toward the lower part of the luminaire. If all of the prisms around the circumference of the reflector ran the full length of the reflector, the prisms would be very large at the largest circumference and very small at the smallest circumference. The requirement that the prisms become broader means that the thickness of the wall of the luminaire must increase toward the larger circumference, which increases the cost and weight of the luminaire.
To make the prism sizes manageable, it is known to provide two or more sets of prisms. One set of prisms, referred to herein as main prisms, runs the entire length, while the other set or sets, referred to herein as transition prisms, begins at the larger circumference and transitions out along the reflector. By this arrangement, fewer prisms are provided at the smaller circumference, and the uniformity of the prism sizes is improved.
A problem presented by this arrangement is that the contours of the main prisms (i.e., those that extend along the entire arc length of the reflector) and of the transition prisms (i.e., those that taper out along the length) differ by the degree of taper incorporated into the transition prisms. The resulting reflector geometry is, therefore, a combination of the two, or more, geometries of the sets of prisms, and the resulting light pattern is the net sum of the light patterns generated by the distinct reflector geometries. A known luminaire having such a reflector is shown in U.S. Pat. No. 4,839,781.
Thus, the design of the reflector to achieve a desired light pattern requires tradeoffs in the different geometries. For example, this configuration may require the main beam to be higher than needed to compensate for a transition beam lower than needed to achieve a beam at the actual desired angle.